In the past — or at least, when I was in elementary school — periods/full stops were followed by two spaces. Lately, it's become more and more common to see just one space. In the modern world, should we still use two spaces between sentences, or is just one okay? Does it depend on the situation? Or are both acceptable, with the choice simply coming down to personal preference?

On the web, if you want two spaces after periods, you need to insert non-breaking space characters or entities (&nbsp;). Since this is such an effort, one space after periods has become the de facto standard on the web. You can type in as many spaces as you please in the HTML, but it will only ever display as a single space.
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nohat♦Sep 4 '10 at 21:21

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I call "false dichotomy" on the whole thing. See @ShreevatsaR 's answer below. There seems to be a confusion between two separate things: A) Should, I as a writer manually enter 2 spaces (i.e. two strokes on the keyboard) between sentences. B) Should there be a larger space between sentences. I agree that the answer to A) should be "no", however, the fact that people preferred 2 spaces over 1 during monospaced fonts seems to indicate that a larger space is more visually appealing. i.e, the answer to B) should be "yes". As to "how to do it...." well that's a separate issue.
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Yossi FarjounOct 30 '10 at 16:11

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@Yossi: I take issue with "the fact that people preferred 2 spaces over 1 during monospaced fonts seems to indicate that a larger space is more visually appealing". It's not a universal "fact", quite the opposite, there's a some missing before the people. Preferring two spaces over one is called English spacing for a reason. You could just as boldly state the exact opposite thing, and you would be just as correct (or wrong): it's a fact that people preferred one space over two during monospaced fonts (which is called the French spacing).
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RegDwigнt♦Nov 3 '10 at 14:41

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@RegDwight: I agree. I intended to say the sentence with "some". However, the point that should be taken that there must be some benefit in this custom. This is a point that I feel was overlooked in the rush to discuss the merits and follies of hitting that space-key twice (which should be an irrelevant point with modern typesetting programs).
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Yossi FarjounNov 3 '10 at 15:24

15 Answers
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Actually, I feel a few of the other answers here (and even the question) are a bit simplistic: there's more to this issue than is indicated by the latest editions of the Chicago Manual of Style or Bringhurst's The Elements of Typographic Style. In lieu of a very long answer, let me point to the (long) Wikipedia articles on exactly this issue:

The traditional typesetters' convention was to use a (single) longer space between sentences than between words. For instance, CMoS 1911 still recommends a 3-em space between words and an em-quad between sentences.

With the introduction of the typewriter (invented in the late 19th century), many typographical niceties were lost: the typewriters produced monospaced (fixed-width) text, and the only choice was between one space and two. Many people felt a single space wasn't sufficient to see the gap between sentences at a glance, so double spacing came into vogue.

Today, with proportional (variable-width) fonts, two spaces is no longer necessary, and can look distractingly too wide. Modern tools allow more choice than between exactly "one space" or two. In particular, TeX and LaTeX have got it right since the 1980s: they typeset a slightly longer space between sentences (though this can be turned off). HTML ignores multiple consecutive spaces anyway. (Sometimes fonts try to be smart and have the period character itself have a wider space following it, but this isn't ideal: there can be periods within a sentence, because of abbreviations etc.)

Even shorter summary (my opinion):
Don't use two spaces unless you're using a fixed-width font like a typewriter. If forced to choose only between one space and two, choose one. But if your typesetting system supports it, have a wider space between sentences.

On your last bullet, I'd say even more to the point is that word processing software is quite capable of detecting the end-of-sentence condition itself and inserting however much space it feels should be there (typically selectable by the user). Fighting it by adding another space yourself is just going to cause you problems.
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T.E.D.Apr 3 '12 at 16:45

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Are you sure, Mr. T.E.D., that's it's not possible to have periods and capital letters in the middle of sentences? It would seem more helpful to the word processing software to have an explicit end-of-sentence indicator, which it can then render correctly, as in TeX.
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endolithSep 21 '12 at 14:29

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+1 for the good and differentiated summary, even though I couldn't disagree more with your opinion. I find TeX's default behaviour most ugly and typing \frenchspacing is among the first things I do for a new document.
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ChristianMar 15 '13 at 15:19

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@T.E.D.: I would argue the opposite: if sentences are marked with a full stop and two spaces, software can separately and consistently control word space and sentence space with 100% accuracy. If sentences are marked with a period and one space, unless other markup is used in ambiguous situations, I see no way a computer could possibly resolve ambiguities. There are cases where a sentence could parse correctly with a period marking a mid-sentence abbreviation or end of sentence, but would mean different things in either case. Only Linotype-style spacing would make such sentences problematical.
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supercatMay 1 '14 at 1:59

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"Richard Smith decided to purchase Acme Inc. Robo-Vacuums and Auto-Cleaner really piqued his interest." Using pre-Linotype typography, the sentence would read one way with a small space after "Inc." and another way with a large space. Using Linotype spacing, I don't know how one would tell which way is correct, and can't imagine a computer could possibly figure it out either unless it knew what Mr. Smith actually did.
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supercatMay 1 '14 at 2:18

However, when justified variable-width type is set for printing it has always been standard to use only one space between sentences. Modern computers produce type that is more like print, and most modern styles call for only one space after a period.

The view at CMOS is that there is no reason for two spaces after a period in published work. Some people, however—my colleagues included—prefer it, relegating this preference to their personal correspondence and notes.

So yes, it basically falls to personal preference, but one space is becoming more and more prevalent.

(It's worth noting that all HTML renderers I know of automatically condense multiple spaces into one, so it would actually take some effort to get the double-space style to render on the web.)

In the nineteenth century, which was a dark and inflationary age in typography and type design, many compositors were encouraged to stuff extra space between sentences. Generations of twentieth-century typists were then taught to do the same, by hitting the space character twice after every period. Your typing as well as your typesetting will benefit from unlearning this quaint Victorian habit. As a general rule, no more than a single space is required after a period, a colon or any other mark of punctuation. Larger spaces (e.g., en spaces) are themselves punctuation.

The rule is usually altered, however, when setting classical Latin and Greek, romanized Sanskrit, phonetics or other kinds of texts in which sentences begin with lowercase letters. In the absence of a capital, a full en space (M/2) between sentences will generally be welcome.

Before I first read this many years ago, I had always used two spaces after every period (as I had been instructed by my middle-school typing teacher). Since then, however, I’ve observed that a single space after a period is used in almost all professionally typeset materials (books, magazines, etc.), and I’ve changed my habits.

I continue to use two spaces after a period when writing in a monospace font, because I feel monospaced text is more readable this way, but I use a single space whenever I’m using a proportional font.

Yes, I believe the user of two spaces on typewriters was for readability.
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Mark CNov 24 '10 at 17:31

There's so much that I learnt from my teachers, that I since learned otherwise later in life. Moral of the story: Always question; always be prepared to learn and more importantly unlearn.
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Craig YoungJul 7 '12 at 12:06

Take a look at books published in the early twentieth century or before. Spaces after sentences were much larger than the spaces after abbreviations. It's ironic that a convention stemming from technical limitations of early Linotypes should be declared as the preferred way of doing things in a medium with no such limitations.
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supercatMay 1 '14 at 2:23

I'm not sure I understand the use of 2 spaces in monowidth fonts and 1 space in proportional. Isn't a space larger in monowidth fonts (ie. an em-width) while it's surely less than an em in proportional fonts?
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Simon EastJun 17 '14 at 6:17

Thank you for being the only post here to mention the en space (and its place in sentences that start with lowercase letters). This hints at the ambiguity of sentences that start with oddly cased words like "iPhone," which IMHO justifies larger spaces ... which should be en spaces rather than two standard spaces, except that the standard space is slightly shorter than a hyphen and shorter still than an n (and two spaces is shorter than an M). I compromise by ignoring all of that and following SWB's rule based on the font family.
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Adam KatzMar 6 at 2:39

It seems that four or so observations have not yet made it into this thread, so let's add them:

Much printed writing (magazines, newspapers) is done with full-width left and right aligned text. This induces a natural (or not) length space after the period which is different in size in each sentence. Here the use of double spaces after periods should be avoided.

The habit of adding an extra space for visible clarity comes mostly from monospaced fonts. Since about all typewriters used to be monospaced and further in history, type setting used to be monospaced too, the extra space had a benefit. Now many (older?) people still have this habit.

In type setting, kerning has always been very important. Kerning is about how much particular letters or punctuation should be apart. A dot should be closer to its preceding letter, double "ff" should be kerned closer than double "dd" for instance and the space before a capital should be larger than a space before a small letter. This automatically introduces a larger space between full stop and first letter in next sentence. Watch closely: a next sentence starting with a quotation character should get a smaller space.

With the introduction of computer type writing (not type setting!) and proportional fonts, a heated debate was and is going on about kerning and the inability to add kerning to fonts or the reluctance of font designers to do so. However, in proportional fonts and online editing, one should leave kerning (and thus: spacing) to the rendering engine, whether to paper or to screen. Screen however has limited possibilities, but with text, kerning is on its way back due to better support in text writers and has always been around for professional type setters (LaTeX and some Adobe products).

Conclusion: double space after a full stop is a cosmetic habit of the typewriting age and before, and should not be used in online writing or proportional / kerned writing.

Great answer, but a minor quibble: "further in history, type setting used to be monospaced too" is false. The very first printed text, Gutenberg's Bible, was exquisitely typeset, using 290 characters (including ligatures, etc). In fact, typesetting was never monospaced, though the quality did deteriorate starting with the proliferation of mechanical typesetting (invented around 1890 and became widespread by the 1970s, I think) — this deterioration was the chief motivation for Knuth to start TeX, to revive the beauty of hand-typeset hot-metal typography.
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ShreevatsaRNov 3 '10 at 16:13

Examples I've seen of typeset material from pre-Linotype days suggest that typesetters would use an extra-wide space after each sentence, and people using typewriters imitated that using two fixed-sized spaces. I don't think many typesetters would have regarded as proper the use of the same size space after a sentence as after a word. I'd suggest that the proper thing would be for computers to recognize sentence spaces as distinct from word spaces, and the most natural way to enter a sentence space would be as two consecutive presses of the space bar.
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supercatMay 1 '14 at 2:32

I used to be a stickler about this in my own writing and when editing others writing as well, but especially as the computer has taken the place of the typewriter in my writing (showing my age a bit) and as the brevity of twitter has influenced my other writing (in a good way, mostly) I've come to see the 2nd space as a waste of space.

One nail in the second space's coffin, for me, was watching this video with Microsoft's ex-typography guru, Bill Hill. He says (transcribed):

I single-space after a period. Double-spacing comes from the days when we had typewriters, and the spacing was kind of gross because typewriters are mono-spaced, right? So that came from those days.

I don't know any typographer who does a double-space after a period, the same way no typographer (or nobody who knows anything about type) would ever use underlining as a way of emphasizing, because you break the word shape. Underlining, again, is one of these things that harks back to the days of typewriter (sic), because underlining was the only kind of emphasis you had. But in good typography, in books or magazines or whatever, people use italics or bold for emphasis, not -- never, never, never, never underlining. Underlining is something you do with a pen when you're reading something yourself 'cause it's an emphasis you can do, or something you do with a typewriter because it's the only one you've got, but never in typeset type, it shouldn't be [used].

I used to be an editor, a proof-reader, at a paper in Scotland called "The Scotsman". "The Scotsman" is kind of a weird thing, I used to be a writer there as well; they did this survey once, there was a reading age required to read various newspapers. So they had "The Sun" -- if you're at the reading age of six, I think, you could read "The Sun". The "London Times" I think you needed a reading age of at least eleven. "The Scotsman" you needed a reading age of thirteen; it was kind of the most erudite paper in the country, and they were always incredibly fussy about the typesetting, the way it was done.

So you kind of learn that stuff, and the double-space is kind of ugly, because a space is not a fixed thing, right? Because, you know, once the application has taken the string of characters and then said "OK, I have to break this string here, I really have to hyphenate this word, or route this word down to the next line. OK, now I have to space out, right? And spacing is one of those things that floats like that, so two spaces is compounding the floating-error of one space, so it's not a good thing to use. That's my story and I'm sticking to it.

+1 for the Bill Hill recording. Someone should transcribe that so it can be useful to more people.
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nohat♦Sep 4 '10 at 21:20

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@nohat I transcribed it here; you're welcome to incorporate it into the answer and delete this comment (I can't edit posts)
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Michael MrozekSep 7 '10 at 0:48

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I've just read that transcript. I see no argument there. Just because it's the "house style" at "The Scotsman" is no reason to impose it on the rest of us.
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Loop SpaceOct 30 '10 at 17:13

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@Andrew the argument is that Mr. Hill is an expert in the art and history of typography and he states (correctly) that in the history of typography the standard has always been one space after periods, the two-space rule only ever applied to typewriters, and that modern word processing is much more akin to classical typography, so the rules that applied to classical typography should be followed in word processing. Furthermore, in his role as an expert in typography he states that the spacing created by double spaces is aesthetically and technically objectionable. “The Scotsman” is irrelevant.
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nohat♦Nov 2 '10 at 18:33

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@nohat: According to cori, this video provides "another nail in the coffin". I've read the transcript and I don't see any argument to rebut there! "Double space" = "underlining" and "it's what we did at 'The Scotsman'" are the best that I can see. Maybe the transcript doesn't carry the full weight of the video so I'm missing something (and @Michael: why a "pre" tag? At least put "white-space: pre-wrap" on it). So I'm afraid that the only argument I can see is "Don't double space because I tell you not to.". Please enlighten me as to what I'm missing.
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Loop SpaceNov 3 '10 at 21:26

As aluded to by one of the respondents, the reason (most likely) that a single space has become common place is due to the fact that HTML won't allow two sequential spaces without a bit of special plumbing, so the second space got lost in a lot of online writing. However now that we are used to it I think it's probably here to stay.

My preference would be for two as I find it a good visual cue of a sentence end.

And the period/full-stop isn't enough of a flag for the sentence's end?
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coriSep 5 '10 at 12:34

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@cori: It's "enough" in the sense that we can make do with it. I agree with Toby that it is nicer if there is more space between sentences than between words. As another answer points out, we sometimes have periods within sentences as well, and I find it conceptually and visually preferable that a sentence-ending period is followed by more space than an abbreviation-ending one.
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John YJan 15 '11 at 0:58

I always thought that an extra space following the period at the end of a sentence and before the start of a new one, as opposed to a single space after a comma or a semicolon, etc. is there to emphasize a slightly longer pause in the rhythm of a language… And typesetting environments like TEX has always handled that elegantly, only wider-spread applications like office productivity suits, etc. do not known, how to handle it correctly.

I dropped my old school days/typewriter habit of typing two spaces after a period after I read about fonts in The Mac is not a Typewriter by Robin Williams. That was way back in 1991.

Robin explained what many have said above: typewriter monospaced fonts needed the second space to compensate for not being able to use proportional fonts. In typesetting with proportional fonts, using one space after a sentence has always been the norm.

It's a wonderful book and she also points out some other stylistic customs to follow in manuscripts for a professional appearance.

As an amazon.com review says:

"What is important for the non professional typist to know [is to] use "smart" quotes, don't space twice after a period, italicize instead of underlining, create a long (em) dash by typing [shift + option + -]"

I find it very odd that there are apparently so many folks who didn't "get the memo" on this even after all these years. heheh

+1 for The Mac is not a Typewriter. That’s the book that convinced me to not do two spaces oh so many years ago. Showed that book to my high school freshman English teacher and he got all mad and said “in MY class you put two spaces after periods”. That class (c. 1994) was the last time I typed two spaces after periods.
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nohat♦Sep 5 '10 at 17:03

Brilliant. I've showed the book to many over the years. Some because they were cranky and didn't believe me (like your teacher) and others because I knew they would appreciate it. ;-)
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LDinSTLSep 6 '10 at 8:23

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The norm for mechanically-typeset type is for a sentence to be followed by a word space. Before that, the norm was for sentences to be followed by a much wider space. Is there any reason a 21st-centry computer should be limited to the typography of an early-1900's Linotype?
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supercatMay 1 '14 at 2:41

This is a good point, but the unfortunate fact is that most readers will not tell the difference even with extra space. (This is the same old "people are idiots; clarity and logic have no place in English" point that keeps coming up in discussions of English.)
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ShreevatsaRMar 8 '11 at 16:33

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With a semicolon or a dash. "Watch it, Mister; Smith is coming." or "Watch it, Mister -- Smith is coming."
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AaronMar 28 '11 at 17:55

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Personally, I don't think the slight extra space really makes it obvious that you intend this to be read as two sentences, i.e. the solution doesn't really solve the problem. The real solution is to follow the standard English rule that titles are normally only abbreviated when they immediately precede a name. We don't write, "Watch it, Mr." We write, "Watch it, mister."
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JayDec 6 '11 at 17:55

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@Kris: I disagree with your assertion that it's "only about the typographics"; typography has to be considered as a part of how the writing is understood, and cannot stand alone. In this case, the typography (one space or two) is not enough to make the writing clear, so rewriting with different punctuation is preferred.
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AaronDec 9 '11 at 16:13

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The term Mr. is only appropriate as part of a name. On the other hand, one could also talk about the popularity of Carl's Jr. Six Dollar Cheeseburgers--pure deliciousness. Use handset-type spacing and there's no ambiguity. Otherwise, who knows whether the popularity applies to the restaurant or just the burgers?
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supercatMay 1 '14 at 2:46

When using a word processor, my father inserts a line break after every line (and two between paragraphs). Needless to say, it messes up word wrapping. Two spaces after each period is similar -- a useless anachronism from the bad old typewriter days.

A well defined font will handle the spacing between characters with kerning, and adding a double space should be unnecessary. If the font you use has such poor kerning that you can't see the period or sentence structure without two spaces, just use a better font. For normal prose, it's just something you shouldn't need to worry about.

Of course, if you're composing concrete poetry or advertising copy, you may have to worry after all.

Others have discussed the typographical issues; I have nothing to add there.

Two spaces are a pain on computers because they require extra, special logic rules for the computer to process correctly. It's not just HTML: two spaces were a pain for the earliest word processors. When the computer sees two spaces, it doesn't know if it's the end-of-sentence rule, or if the user is trying to make columns line up or do some other special formatting. It requires some extra logic when you get to the end of a line: if a line ends with the end of a sentence, we don't want to leave an extra gap at the end of that line, making it uneven with lines above and below, and we don't want to carry the space down to the next line. Etc. Of course writers of word processing software have long since come up with solutions to these problems, but in the early days it could be an issue.

Yes, I was taught this rule in typing class too, back in the 1970's. I think it was simply an unnecessary, nuisance rule back then, and more so now. It's one of those rules that is mostly followed because it is a rule rather than because there is any good reason for it.

Possibly because of the first sentence? Or maybe that your argument is unattributed?
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user867May 27 '13 at 4:24

If a program is supposed to set type to match pre-Linotype conventions, it must be able to distinguish sentence breaks from abbreviations. Such processing would be easiest if sentences were followed by a "sentence space" character, but substituting such a character for a pair of spaces after a punctuation mark is pretty trivial. There is no other way to handle such things 100% consistently without requiring markup in ambiguous cases, and I can't think of any markup easier than simply using two spaces.
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supercatMay 1 '14 at 2:51

@supercat Interesting thought. So okay, "etc. " (one space) is an abbreviation but "blah blah. " (two spaces) is the end of a sentence. I suppose if you're doing full justification, for example, you'd want to add more space after a sentence than after an abbreviation. That's not 100%, though, as someone could use spaces to line up columns, for example.
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JayMay 1 '14 at 12:05

@Jay: In general, if text is used to line up columns it will be pretty obvious that they're going beyond the normal single and double spaces. In any case, I don't know how a text-to-speech engine would know how to distinguish "Fred bought a Carl's Jr. Double Western Bacon Cheeseburgers rock." from "Fred bought a Carl's Jr. _ Double Western Bacon Cheeseburgers rock." without a space to indicate whether Fred bought a rock or a restaurant.
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supercatMay 1 '14 at 12:34